Problem: On the first 3 quizzes of his physics class, Christopher got an average score of 77. What does he need on the next quiz to have an overall average of 80?
Let his score on the next quiz be $x$ The sum of all of his scores is then $3 \cdot 77 + x$ The same sum must also be equal to $4 \cdot 80$ Solve: $x = 4 \cdot 80 - 3 \cdot 77 = 89$.